Integral Control

Integral (reset) action is the result of an integration of controller error with time. With integral action, controller output is proportional to both the size and duration of the error. As long as a deviation from setpoint exists, the controller continues to drive its output in the direction that reduces the deviation. The rate of change of controller output is proportional to the magnitude of the error. Figure 300-11 illustrates the open loop response of integral action.

Integral action is normally used in conjunction with proportional action; it is rarely used by itself. Integral action is quantified as the time (the reset time) required to change controller output by an amount equal to the change caused by proportional action. In other words, it is the time required to repeat the contribution of the proportional action.

On some controllers, integral settings are in repeats, meaning repeats per minute; on others, settings are in minutes, meaning minutes per repeat. One setting is the reciprocal of the other; decreasing the integral time increases the amount of integral action.